Building up Complexity from Strips and Sheets: The Electronic Structure of the La12Mn2Sb30 Alloy

The bonding in the relatively complex La12Mn2Sb30 alloy structure is analyzed with a retrotheoretical/building up process, implemented through a molecular orbital analysis of the various sublattices and the composite structure. In the antimony part of La12Mn2Sb30 there are three relatively noninteracting networks: a three-dimensional Sb20 sublattice, an Sb6 strip, and a onedimensional array of isolated Sb atoms (of Sb4 stoichiometry). A Zintl-type approach, modified for the clearly hypervalent nature of locally linear and square-planar Sb environments, leads to an initial partitioning of the electrons among the Sb sublattices; this electron counting eventually turns out to be in reasonable agreement with extended Hückel calculations. The electronic structure of the three-dimensional Sb20 sublattice in La12Mn2Sb30 is derived theoretically from a two-dimensional square Sb sheet through first kinking the square sheet at every fifth diagonal line and then stacking the sheets, with Sb–Sb bond formation, into the third dimension. For the Sb6 strips a secondorder Peierls-type distortion of symmetrical vertex-sharing rhombi leads to the slightly asymmetrical strip structure observed. The d-block splitting of the Mn ions (in an unusual bicapped tetrahedral Sb environment) is described by a molecular model; arguments are given for localized bonding at Mn. There are significant La–Sb network interactions. The ability of the Sb networks in this structure to act as electron reservoirs is supported by our calculations. ( 1998 Academic Press